FIG. 1 illustrates a prior porous silicon emitter 100. The prior porous silicon emitter 100 is a diode structure that includes a heavily doped n+ silicon (Si) substrate 103 that serves as an electron injection layer, an optional ohmic contact 105 in electrical contact with the substrate 103, an active porous silicon (Si) layer 101 formed on the substrate 103, and an electrode 107 formed on the active porous silicon layer 101 and in electrical communication with the electrode 107. When the electrode 107 is biased positively relative to the substrate 103, a diode current Id, supplied by a voltage source V1, passes through the active layer 101 and the substrate 103. A fraction of the diode current Iθ, is injected into a vacuum region (not shown) above the electrode 107 and is collected by a collector electrode 115 that is positioned opposite the electrode 107. The collector electrode 115 is biased positively relative to the electrode 107 by a voltage source V2 to extract electrons e− that are emitted by the electrode 107. The electrodes (107, 115) and the ohmic contact 105 can be made from an electrically conductive material such as gold (Au) or aluminum (Al).
One disadvantage of the prior porous silicon emitter 100 is that the active porous silicon (Si) layer 101 has a high porosity that results in a high series contact resistance RC between the electrode 107 and the active porous silicon (Si) layer 101. The resistance Rc is comparable with or even larger than the resistance of the active porous silicon (Si) layer 101 at high voltage. Consequently, the high series contact resistance Rc creates an undesirable/unintentional voltage drop between the active layer 101 and the electrode 107 that reduces an electron emission efficiency of the porous silicon emitter 100.
Moreover, the high series contact resistance Rc results in a higher power consumption and higher power dissipation (waste heat). This tends to reduce the useful life time of the emitter 100. In battery powered applications it is desirable to reduce power consumption so that battery life and operating time are extended. Furthermore, it is desirable to reduce the amount of waste heat generated by a system because thermal management systems such as fans and heat sinks add to system cost, weight, and complexity.
A second disadvantage of the prior porous silicon emitter 100 is that the contact resistance Rc causes the diode and emission current to saturate at high bias voltages supplied by V1. It is desirable to have the electron emission current increase with increasing voltage levels. However, if saturation occurs the electron emission current peaks and does not increase with increasing voltage.
Finally, another disadvantage of the prior porous silicon emitter 100 is that the active porous silicon (Si) layer 101 has a high contact resistance with the electrode 107 that results in a reduction in electron emission efficiency.
Therefore, there exists a need for a porous silicon emitter that reduces the series contact resistance between an active porous silicon layer and an electrode of the porous silicon emitter. There is also a need for a porous silicon emitter that can operate at lower voltages thereby reducing power consumption and generation of waste heat. Furthermore there is a need for a porous silicon emitter that does not saturate at higher voltages so that high emission currents and efficiency are obtainable at those higher voltages.